1. Field of the Invention
The invention relates to an apparatus for detecting deposition on a surface, and in particular ice deposition on a cooling body, heat pump or the like.
2. State of the Prior Art
If a body has a temperature below the freezing temperature of vapour present in the ambient air, the vapour will freeze on said body. Thus the cooling body or evaporator of a refrigerating device will be covered, after some time, with an ice layer screening said body from the surroundings, which will cause the heat transfer to be substantially reduced. Therefore the cooling body is to be defrosted at regular intervals.
The longer one waits with defrosting, the longer the defrosting will last, and, moreover, the device will have operated at a lower heat-transfer efficiency during a longer period preceding the defrosting. If defrosting is initiated shortly after the beginning of the ice deposition, a relatively short defrosting period will suffice, but one should avoid, then, to disturb the temperature in the vicinity too often by defrosting.
Apparatuses for detecting ice deposition are known. In an apparatus known from GB Pat. No. 1 013 182 use is made of a vibrator adapted to be vibrated mechanically by means of an electrical vibration source, said vibrator being exposed to ice deposition, and forming itself a part of the vibration circuit. Means are present for detecting the changes in the vibration frequency of the vibrator when ice is being deposited thereon, causing the mass and, particularly, the stiffness and therefore the resonant frequency thereof to change. The change in the resonant frequency of the vibrator can be followed, and, as soon as the ice deposition limit at which defrosting should begin is reached, a signal can be derived from the frequency change by means of which means for removing the ice deposit can be switched on.
A draw-back of measuring changes in the resonant frequency is that the variation of the frequency with the thickness of the ice layer is small, and these frequencies will differ from one vibrator to an other, and, moreover, will be dependent on the temperature. This not only requires a precise adjustment of each apparatus, but also complicated compensation means for the temperature shift.